# Source code for burnman.eos.birch_murnaghan_4th

from __future__ import absolute_import
# This file is part of BurnMan - a thermoelastic and thermodynamic toolkit for the Earth and Planetary Sciences
# Copyright (C) 2012 - 2017 by the BurnMan team, released under the GNU
# GPL v2 or later.

import numpy as np
import scipy.optimize as opt
from . import equation_of_state as eos
from ..utils.math import bracket
import warnings

def bulk_modulus_fourth(volume, params):
"""
compute the bulk modulus as per the fourth order
birch-murnaghan equation of state.  Returns bulk
modulus in the same units as the reference bulk
modulus.  Pressure must be in :math:[Pa].
"""

x = params['V_0'] / volume
f = 0.5 * (pow(x, 2. / 3.) - 1.0)

Xi = (3. / 4.) * (4. - params['Kprime_0'])
Zeta = (3. / 8.) * ((params['K_0'] * params['Kprime_prime_0']) + params[
'Kprime_0'] * (params['Kprime_0'] - 7.) + 143. / 9.)

K = (5. * f * pow((1. + 2. * f), 5. / 2.) * params['K_0'] * (1. - (2. * Xi * f) + (4. * Zeta * pow(f, 2.)))) + \
(pow(1. + (2. * f), 7. / 2.) * params['K_0'] * (
1. - (4. * Xi * f) + (12. * Zeta * pow(f, 2.))))

return K

def volume_fourth_order(pressure, params):
func = lambda x: birch_murnaghan_fourth(
params['V_0'] / x, params) - pressure
try:
sol = bracket(func, params['V_0'], 1.e-2 * params['V_0'])
except:
raise ValueError(
'Cannot find a volume, perhaps you are outside of the range of validity for the equation of state?')
return opt.brentq(func, sol, sol)

def birch_murnaghan_fourth(x, params):
"""
equation for the fourth order birch-murnaghan equation of state, returns
pressure in the same units that are supplied for the reference bulk
modulus (params['K_0'])
"""

f = 0.5 * (pow(x, 2. / 3.) - 1.0)
Xi = (3. / 4.) * (4. - params['Kprime_0'])
Zeta = (3. / 8.) * ((params['K_0'] * params['Kprime_prime_0']) + params[
'Kprime_0'] * (params['Kprime_0'] - 7.) + 143. / 9.)

return 3. * f * pow(1. + 2. * f, 5. / 2.) * params['K_0'] * (1. - (2. * Xi * f) + (4. * Zeta * pow(f, 2.))) + params['P_0']

[docs]class BM4(eos.EquationOfState):

"""
Base class for the isothermal Birch Murnaghan equation of state.  This is fourth order in strain, and
has no temperature dependence.
"""

[docs]    def volume(self, pressure, temperature, params):
"""
Returns volume :math:[m^3] as a function of pressure :math:[Pa].
"""
return volume_fourth_order(pressure, params)

[docs]    def pressure(self, temperature, volume, params):
return birch_murnaghan_fourth(volume / params['V_0'], params)

[docs]    def isothermal_bulk_modulus(self, pressure, temperature, volume, params):
"""
Returns isothermal bulk modulus :math:K_T :math:[Pa] as a function of pressure :math:[Pa],
temperature :math:[K] and volume :math:[m^3].
"""
return bulk_modulus_fourth(volume, params)

[docs]    def adiabatic_bulk_modulus(self, pressure, temperature, volume, params):
"""
Returns adiabatic bulk modulus :math:K_s of the mineral. :math:[Pa].
"""
return bulk_modulus_fourth(volume, params)

[docs]    def shear_modulus(self, pressure, temperature, volume, params):
"""
Returns shear modulus :math:G of the mineral. :math:[Pa]
"""
return 0.

[docs]    def entropy(self, pressure, temperature, volume, params):
"""
Returns the molar entropy :math:\mathcal{S} of the mineral. :math:[J/K/mol]
"""
return 0.

[docs]    def molar_internal_energy(self, pressure, temperature, volume, params):
"""
Returns the internal energy :math:\mathcal{E} of the mineral. :math:[J/mol]
"""
x = np.power(volume/params['V_0'], -1./3.)
x2 = x*x
x4 = x2*x2
x6 = x4*x2
x8 = x4*x4

xi1 = 3.*(4. - params['Kprime_0'])/4.
xi2 = 3./8.*(params['K_0'] *
params['Kprime_prime_0'] +
params['Kprime_0'] *
(params['Kprime_0'] - 7.)) + 143./24.

intPdV = (-9./2. * params['V_0'] * params['K_0'] *
((xi1 + 1.)*(x4/4. - x2/2. + 1./4.) -
xi1*(x6/6. - x4/4. + 1./12.) +
xi2*(x8/8 - x6/2 + 3.*x4/4. - x2/2. + 1./8.)))

return - intPdV + params['E_0']

[docs]    def gibbs_free_energy(self, pressure, temperature, volume, params):
"""
Returns the Gibbs free energy :math:\mathcal{G} of the mineral. :math:[J/mol]
"""
# G = int VdP = [PV] - int PdV = E + PV

return self.molar_internal_energy(pressure, temperature, volume, params) + volume*pressure

[docs]    def molar_heat_capacity_v(self, pressure, temperature, volume, params):
"""
Since this equation of state does not contain temperature effects, simply return a very large number. :math:[J/K/mol]
"""
return 1.e99

[docs]    def molar_heat_capacity_p(self, pressure, temperature, volume, params):
"""
Since this equation of state does not contain temperature effects, simply return a very large number. :math:[J/K/mol]
"""
return 1.e99

[docs]    def thermal_expansivity(self, pressure, temperature, volume, params):
"""
Since this equation of state does not contain temperature effects, simply return zero. :math:[1/K]
"""
return 0.

[docs]    def grueneisen_parameter(self, pressure, temperature, volume, params):
"""
Since this equation of state does not contain temperature effects, simply return zero. :math:[unitless]
"""
return 0.

[docs]    def validate_parameters(self, params):
"""
Check for existence and validity of the parameters
"""

if 'E_0' not in params:
params['E_0'] = 0.
if 'P_0' not in params:
params['P_0'] = 0.

# If G and Gprime are not included this is presumably deliberate,
# as we can model density and bulk modulus just fine without them,
# so just add them to the dictionary as nans
if 'G_0' not in params:
params['G_0'] = float('nan')
if 'Gprime_0' not in params:
params['Gprime_0'] = float('nan')

# Check that all the required keys are in the dictionary
expected_keys = ['V_0', 'K_0', 'Kprime_0']
for k in expected_keys:
if k not in params:
raise KeyError('params object missing parameter : ' + k)

# Finally, check that the values are reasonable.
if params['P_0'] < 0.:
warnings.warn('Unusual value for P_0', stacklevel=2)
if params['V_0'] < 1.e-7 or params['V_0'] > 1.e-3:
warnings.warn('Unusual value for V_0', stacklevel=2)
if params['K_0'] < 1.e9 or params['K_0'] > 1.e13:
warnings.warn('Unusual value for K_0', stacklevel=2)
if params['Kprime_0'] < 0. or params['Kprime_0'] > 10.:
warnings.warn('Unusual value for Kprime_0', stacklevel=2)
if params['Kprime_prime_0'] > 0. or params['Kprime_prime_0'] < -10.:
warnings.warn('Unusual value for Kprime_prime_0', stacklevel=2)